It is well known that regulating the exposure of oxygen-sensitive products to oxygen maintains and enhances the quality and "shelf-life" of the product. For instance, by limiting the exposure of oxygen sensitive food products to oxygen in a packaging system, the quality or freshness of food is maintained, spoilage reduced, and the food shelf life extended. In the food packaging industry, several means for regulating oxygen exposure have already been developed. These means include modified atmosphere packaging (MAP) and oxygen barrier film packaging.
For packaging material used in gable top or rectangular cartons, a coated paper or cardboard stock is often used. The coating for the paper or cardboard stock is usually a polymer-based resin, such as polyethylene, which can be applied to the paper or paperboard stock by extrusion coating or laminating. Such a coating serves not only to make the packaging material waterproof, but can also serve as an oxygen barrier.
In one known example of such an extrusion coated paper packaging material, the extrusion coating composition is comprised of greater than 20 and less than 98 weight percent of a high pressure low density polyethylene homopolymer and/or copolymer and greater than 2 and less than 80 weight percent of at least one linear low density ethylene hydrocarbon copolymer.
In an example of such a resin coated packaging material specifically designed to have enhanced oxygen barrier qualities, an additional layer of polyamide is added to the low density polyethylene laminated paperboard. In a similar example, an additional layer of heat-sealable ethylene vinyl alcohol copolymer is added to the low density polyethylene laminated paperboard.
One method currently being used for regulating oxygen exposure is "active packaging", whereby the package containing the food product has been modified in some manner to regulate the food's exposure to oxygen. One form of active packaging uses oxygen-scavenging sachets which contain a composition which scavenges the oxygen through oxidation reactions. One type of sachet contains iron-based compositions which oxidize to their ferric states. Another type of sachet contains unsaturated fatty acid salts on a particulate adsorbent. Yet another sachet contains metal/polyamide complex. However, one disadvantage of sachets is the need for additional packaging operations to add the sachet to each package. A further disadvantage arising from the iron-based sachets is that certain atmospheric conditions (e.g., high humidity, low CO.sub.2 level) in the package are sometimes required in order for scavenging to occur at an adequate rate. Further, the sachets can present a problem to consumers if accidentally ingested.
Another means for regulating exposure of a packaged product to oxygen involves incorporating an oxygen scavenger into the packaging structure itself. A more uniform scavenging effect through the package is achieved by incorporating the scavenging material in the package instead of adding a separate scavenger structure (e.g., a sachet) to the package. This may be especially important where there is restricted airflow inside the package. In addition, incorporating the oxygen scavenger into the package structure provides a means of intercepting and scavenging oxygen as it permeates the walls of the package (herein referred to as an "active oxygen barrier"), thereby maintaining the lowest possible oxygen level in the package. Limited success has been achieved in incorporating oxygen scavenging material into the walls of packages for various types of food.
One attempt to prepare an oxygen-scavenging wall involves the incorporation of inorganic powders and/or salts. However, incorporation of these powders and/or salts causes reduction of the wall's optical transparency, discoloration after oxidation, and reduced mechanical properties such as tear strength. In addition, these compounds can lead to processing difficulties, especially when fabricating thin films. The oxidation products, which can be absorbed by food in the container, typically would not have FDA approval for human consumption.
Some oxygen scavenging systems produce an oxygen-scavenging wall. This is done by incorporating a metal catalyst-polyamide oxygen scavenging system into the package wall. Through catalyzed oxidation of the polyamide, the package wall regulates the amount of oxygen which reaches the interior volume of the package (active oxygen barrier) and has been reported to have oxygen scavenging rate capabilities up to about 5 cubic centimeters (cc) oxygen per square meter per day at ambient conditions. However, this system suffers from significant disadvantages.
One particularly limiting disadvantage of polyamide/catalyst materials can be a low oxygen scavenging rate. Adding these materials to a high-barrier package containing air can produce a package which is not generally suitable for creating the desired internal oxygen level.
There are also disadvantages to having the oxygen-scavenging groups in the backbone or network structure in this type of polyamide polymer. The basic polymer structure can be degraded and weakened upon reaction with oxygen. This can adversely affect physical properties such as tensile or impact strength of the polymer. The degradation of the backbone or network of the polymer can further increase the permeability of the polymer to those materials sought to be excluded, such as oxygen.
Moreover, polyamides previously used in oxygen scavenging materials, such as MXD6, are typically incompatible with thermoplastic polymers used in most plastic packaging walls, such as ethylene-vinyl acetate copolymers and low density polyethylene. Even further, when such polyamides are used by themselves to make a package wall, they may result in inappropriately stiff structures. They also incur processing difficulties and higher costs when compared with the costs of thermoplastic polymers typically used to make flexible packaging. Even further, they are difficult to heat seal. Thus, all of these are factors to consider when selecting materials for packages, especially multi-layer flexible packages and when selecting systems for reducing oxygen exposure of packaged products.
Another approach to scavenging oxygen is an oxygen-scavenging composition comprising an ethylenically unsaturated hydrocarbon and a transition metal catalyst. Ethylenically unsaturated compounds such as squalene, dehydrated castor oil, and 1,2-polybutadiene are useful oxygen scavenging compositions, and ethylenically saturated compounds such as polyethylene and ethylene copolymers are useful as diluents. Compositions utilizing squalene, castor oil, or other such unsaturated hydrocarbon typically have an oily texture as the compound migrates toward the surface of the material. Further, polymer chains which are ethylenically unsaturated in the backbone would be expected to degrade upon scavenging oxygen, weakening the polymer due to polymer backbone breakage, and generating a variety of off-odor, off-taste by-products.
Oxygen scavenging layers extruded or laminated onto the surface of paperboard stock have been tried with limited success. In one of these examples, the oxygen scavenging layer is an ethylenically unsaturated hydrocarbon and a transition metal catalyst. Other known examples of an oxygen scavenging layer that can be coated onto the surface of paper board stock and which furthermore retain oxygen scavenging capabilities at low temperatures are atactic-1,2-polybutadiene, EPDM rubbers, polyoctenamer, and 1,4-polybutadiene.
An oxygen-scavenging composition comprising a blend of a first polymeric component comprising a polyolefin is known, the first polymeric component having been grafted with an unsaturated carboxylic anhydride or an unsaturated carboxylic acid, or combinations thereof, or with an epoxide; a second polymeric component having --OH, --SH, or --NHR.sup.2 groups where R.sup.2 is H, C.sub.1 -C.sub.3 alkyl, substituted C.sub.1 -C.sub.3 alkyl; and a catalytic amount of metal salt capable of catalyzing the reaction between oxygen and the second polymeric component, the polyolefin being present in an amount sufficient so that the blend is non phase-separated. A blend of polymers is utilized to obtain oxygen scavenging, and the second polymeric component is preferably a polyamide or a copolyamide such as the copolymer of m-xylylene-diamine and adipic acid (MXD6).
Other oxidizable polymers recognized in the art include "highly active" oxidizable polymers such as poly(ethylene-methyl acrylate-benzyl acrylate), EBZA, and poly(ethylene-methyl acrylate-tetrahydrofuryl acrylate), EMTF, as well as poly(ethylene-methyl acrylate-nopol acrylate), EMNP. Blends of suitable polymers are also acceptable, such as a blend of EMTF and poly-d-limonene. Although effective as oxygen scavengers, these polymers have the drawback of giving off a strong odor before oxygen scavenging and large amounts of volatile byproducts before and after oxygen scavenging.
Also known are oxygen-scavenging compositions which comprise a transition-metal salt and a compound having an ethylenic or polyethylenic backbone and having allylic pendent or terminal moieties which contain a carbon atom that can form a free radical that is resonance-stabilized by an adjacent group. Such a polymer needs to contain a sufficient amount and type of transition metal salt to promote oxygen scavenging by the polymer when the polymer is exposed to an oxygen-containing fluid such as air. Although effective as oxygen scavengers, upon oxidation, it has been found that allylic pendent groups on an ethylenic or polyethylenic backbone tend to generate considerable amounts of organic fragments. It is believed that this is a result of oxidative cleavage. These fragments can interfere with the use of allylic pendent groups as oxygen scavengers in food packaging by generating compounds that can affect taste and odor of the packaged products.
The present invention solves many of the problems of the prior art encountered when oxygen scavenging material has been incorporated into packaging materials. In various specific embodiments, the present invention solves many of the particular problems encountered with incorporating oxygen scavenging material into the structure of food packaging material such as paperboard stock for gable-top or rectangular cartons.